UBV photometric system

The UBV photometric system, also called the Johnson system or Johnson-Morgan system, is an early-adopted, widely-used photometric system and is a system generally assumed if no other system is specified or implied. It specifies U, B, and V passbands, which in turn specifying B-V and U-B color indices. The magnitude calibration is chosen to give Vega's V apparent magnitude zero (the exact calibration determined using a number of stars, i.e., not simply depending upon Vega), and so that the B-V and U-B color indexes are zero for stars of Vega's spectral type, i.e., A​0 V stars (with compensation for reddening).

The UBVRI photometric system, i.e., Johnson-Cousins system is an extension with red and infrared passbands, R band and I band.